Login plugin: Enable configurable Admin-specific login redirects

When Admin users in a specific group have limited access (e.g., only to the Careers Flex Directory), automatically redirect them to the admin/careers page upon login to /admin, rather than requiring them to manually click "Careers" after each login.
```
redirect_after_login: true # Path to redirect to after a successful login
redirect_after_logout: true # Path to redirect to after a successful logout
session_user_sync: false # Sync session user with changes in stored user file
site_host: # Site host to use for email links. Enhances security by ensuring links are to the correct host (e.g. https://foo.com)

route: '/login' # Specific route for Login page (default is '/login')
route_after_login: '/admin/pages' # Route to go to after login if enabled
```
Another example is redirecting content managers directly to admin/pages instead of requiring them to click "Pages" after each login.

The issue names no files, tests, or entry points. Start by locating the admin login handler and configuration handling for route_after_login, then trace how user groups and admin destinations are resolved. Done means eligible admin users are redirected to their configured destination after login while existing login behavior remains unchanged.

Written by the indexing model from the issue text.